The present invention generally relates to electronic amplifiers. More particularly the present invention relates to linear variable gain traveling wave amplifiers.
Distributed amplifiers in general and traveling wave amplifiers (TWA) in particular are known in the art and are used in a variety of applications. FIG. 1 illustrates a prior art distributed amplifier 100 having an input impedance matching circuit 110, a output impedance matching circuit 120, and a plurality of amplification stages 132, 134, and 136. Each amplification stage, for example, the stage 132 includes an amplification field effect transistor (FET) 140 and a gain control FET 150.
In many microwave and mm-wave radio applications, gain control function is required to realize certain receiver or transmitter functions. For those applications, variable gain amplifiers have been using conventional variable gain TWA uses cascaded FET structure to obtain gain control function shown in FIG. 1. To control the amplifier gain, DC voltage for the Vcontrol terminal is changed.
In the illustrated example, the input impedance matching circuit 110 receives input voltage embodying input signal, Vin, and distributes the input signal to the gate 142 of the amplification FET 140. The amplification FET 140 is DC biased by ground on its source 144 and the output voltage Vout at its drain 146. The input signal Vin (now at the gate 142 of the amplification FET 140) controls drain-source current, Ids-140, between the drain 146 and the source 144 of the amplification FET 140, thereby controlling the output voltage embodying an output signal, Vout. Thus, the Ids-140 depends on the DC biasing of the drain 146 (of the amplification FET 140) relative to its source 144 (the DC biasing of drain-source is referred to as Vds) and the voltage at its gate relative to the voltage at the source (the DC biasing of gate-source is referred to as Vgs). In the illustrated example, the source is grounded and that the gate is connected to Vin; thus, Vgs is Vin.
The amplification FET 140 is serially connected to the gain control FET 150 in the illustrated cascade manner. The gain control FET receives a gain control signal, Vcontrol, at its gate 152. Vcontrol determines the drain-source current, Ids-150, between the drain 156 and the source 154 of the gain control FET 150. Because Vcontrol is a constant signal, and because amplification FET 140 is serially connected to the gain control FET 150, the effect of the gain control FET 150 is to limit the Ids-140 by preventing the full output voltage Vout from reaching the drain 146 of the amplification FET 140. A DC blocking capacitor 158 is used to isolate the Vcontrol signal from ground.
That is, changes in Vcontrol affect Ids-150 which, in turn, affects the DC biasing of the amplification FET 140 by changing the voltage at the drain 146 of the amplification FET 140. Further, the limitation of the Ids-150 limits the Ids-140. Changing of the DC biasing of the amplification FET 140 prevents the amplification FET 140 from operating at its linear portion of its operating characteristic curve. This is illustrated in FIG. 2.
FIG. 2 illustrates a graph illustrating the operating characteristic curve 200 representing operating characteristics of the amplification FET 140 as the drain-source current, Ids, in response to the gate-source voltage Vgs. As illustrated, a middle portion 202 of the operating characteristic curve 200 shows that at some preferred range of Vgs, the amplification FET 140 responds to the changes in the Vgs (due to Vin signal at the gate 144) with corresponding nearly linear changes of its Ids.
Also illustrated is another portions 204 of the operating characteristic curve 200 which show that at this range of Vgs, the amplification FET 140 responds to the changes in the Vgs (due to Vin signal at the gate 144) with a corresponding non-linear changes in its Ids. Such non-linear response is not desired because the non-linear response likely will lead to distortion of the input signal Vin as it is amplified by the distributed amplifier 100. As illustrated, generally, the FET 140 shows poor linearity when its drain current is low. The conventional variable gain TWA shows poor linearity when control the amplifier gain low.
In most applications of distributed amplifiers, it is desirable to have distortion free, linear responses to its input signals while still being able to control the gain of the distributed amplifier with a gain control signal, Vcontrol.